Today, checkpoint security systems in public places like airports or government buildings typically include some combination of an imaging test, a metal detector, and a chemical test. The chemical test usually uses the table-top explosive trace detection (ETD) machine in which a swab or an air sample is taken from an object (e.g., a bag) and tested for trace explosive materials.
Unfortunately, the security check systems that are currently in use are not as reliable as they could be. For example, the X-ray tests identify threatening items based on object densities, and many innocuous objects have densities that are similar to those of some threatening items. Naturally, the rate of false-negative is high. With the imaging test involving X-ray or CT-scan, the accuracy of the test depends largely on the alertness and judgment of a human operator who reviews the images as the bags are scanned. While several systems include automatic visual classification of suspect items, reliance on human alertness and judgment still plays a major role in these systems. Due to distractions, fatigue, and natural limitation on human attention span, a check system that relies so heavily on human judgment cannot reach an optimal level of accuracy. Moreover, because imaging test relies heavily on the visualization of objects being tested, a passenger can disguise or hide a harmful threatening item and avoid detection by the imaging test.
Attempts have been made to increase the accuracy of a checkpoint security system by using a combination of tests, such as imaging, metal detector, and a chemical test. Typically, the tests are performed by utilizing three separate equipments and placing them next to one another. Objects are tested by the separate equipments separately and sequentially, one test after another. For example, an airport security system may employ an X-ray image test and subject only bags that are indicated as being suspect by the X-ray image test to a chemical test. Similarly, as for passengers, they may first be asked to pass through a preliminary metal detection portal, and be subjected to a more stringent metal detector test performed by a human operator only if an alarm is raised by the preliminary portal test.
A problem with this type of serial/sequential combination of tests is that the overall accuracy depends heavily on the accuracy of each individual test, and in some cases on the accuracy of the first test. For example, if the chemical test is not used unless a bag fails the X-ray imaging test, the use of the chemical test is only helpful if the X-ray imaging test accurately identifies the suspect bags. If the operator reviewing the X-ray images misses a potential threatening item, the fact that the chemical test is readily available does not change the fact that the potential threatening item passed through the security system.
While using multiple tests on every passenger and luggage would be an obvious way to enhance the accuracy of security checks, such solution is not practical because it would result in passengers spending an inordinate amount of time going through the security checks. Moreover, such system would be prohibitively costly. For a practical implementation, the accuracy of the security check tests is balanced by—and compromised by—the need to move the passengers through the system at a reasonable rate. Also, if a test that yields a high rate of false-positives like the X-ray test is used as the first test, the flow of passengers is unnecessarily slowed down because many bags that do not contain a threatening item would have to be subjected to the second test.
A system and method for moving the passengers through a security checkpoint at a reasonable rate without compromising the accuracy of the security check tests is desired.